Casaco vs. sobretudo
In Portuguese, both casaco and sobretudo refer to outerwear, but they are not interchangeable. The main distinction lies in their length, weight, and the context in which they are worn, with casaco being a general term and sobretudo describing a very specific type of long coat.
Casaco
A1This is the most common and general Portuguese word for coat or jacket. A casaco can be of any length (from waist to thigh level), style, or material. It is a broad term that covers everything from a light spring jacket to a heavy winter coat.
Vou levar um casaco de malha, pois pode ficar frio mais tarde.
(I'm going to take a cardigan, as it might get cold later.)
Ele comprou um casaco de inverno muito quente para a neve.
(He bought a very warm winter coat for the snow.)
O meu casaco de ganga é perfeito para o outono.
(My denim jacket is perfect for the autumn.)
Ela precisa de um casaco impermeável por causa da chuva.
(She needs a waterproof coat because of the rain.)
Preciso de passar a ferro o casaco do meu fato.
(I need to iron my suit jacket.)
Sobretudo
B2This word specifically means overcoat. A sobretudo is a long, formal, and typically heavy coat that extends to the knees or lower. Its name literally means over-everything (sobre + tudo), as it is designed to be worn over other layers of clothing, such as a suit or another jacket, for protection against severe cold.
No inverno de Nova Iorque, é essencial usar um sobretudo.
(In the New York winter, it is essential to wear an overcoat.)
O empresário vestia um elegante sobretudo de caxemira por cima do fato.
(The businessman wore an elegant cashmere overcoat on top of his suit.)
O sobretudo dela era tão comprido que quase arrastava no chão.
(Her overcoat was so long that it almost dragged on the floor.)
Ele pendurou o sobretudo molhado assim que entrou em casa.
(He hung up his wet overcoat as soon as he got home.)
A personagem do filme usava sempre um sobretudo escuro e um chapéu.
(The character in the movie always wore a dark overcoat and a hat.)
Summary
In essence, casaco is the general word for any coat or jacket. Sobretudo is a specific term for a long, heavy overcoat worn over other garments, typically in very cold weather and often in more formal settings. You can say that every sobretudo is a type of casaco, but most casacos are not sobretudos.